1. Field of the Invention
The present invention relates to actuators and more particularly relates to highly efficient miniature actuators for use with probes employed in conjunction with electrical circuit testing on miniature devices such as integrated circuit chips, wherein the probe must move in a predetermined, accurate and rapid fashion into engagement with the electrical circuit under accurate and rapid fashion into engagement with the electrical circuit under test.
2. Description of Related Art
New technology has increased the density of very large scale integration electronic circuits (VLSI) which requires a reliable probing machine to perform fast and accurate electrical testing. For example, a single substrate may contain as many as 150,000 test points, and the circuits formed between selected points may require probing for different electrical properties at different stages of the manufacturing process.
One of the crucial components of the probing machine is the probe positioning actuator. The actuator moves the probe tip in a vertical direction (Z-Axis) in order to make contact with the test pads of the circuitry to perform the desired or selected test. As can be imagined from the forgoing, competitive probe positioning actuators should be reliable, have low or no maintenance and if required be easy to maintain, should preferably have low or no wear parts or parts that are subject to fatigue, and should have excellent repeatability of position and capable of moving at high speeds without damage to the substrate (semi-conductor chip) under test.
Machines for probing and testing electrical circuits in integrated circuit chips usually include a stationary base member and an X-Y table mounted thereon for movement relative to the base member. A fixture or jig is conventionally provided attached to the table, the fixture being employed to accurately hold, position and vertically elevate the chip into contact with one or more probes for chip monitoring and test. In this arrangement the probes are fixedly connected to the base member, or if monitoring (testing) of the chip is to occur on opposite sides thereof simultaneously, other jig and fixture means are provided for swinging the testing probes into position. Because of the mass of the fixture or jig, (as compared with the mass of the probes), chip movement into engagement with the probe(s) is slow. Moreover, with fixed probes, it is extremely difficult to adjust the probe tips to ensure approximate uniformity of pressure when the chip engages the probes. Various means have been provided in an attempt to ensure substantially uniform probing force. These include planarization of both the probe tips and/or the surface of the chip under test.
Other ways, such as pre-loading the probe arms, ensuring a constant deflection distance of the probe arms etc. have been partially successful but have resulted in differences when the chip surface is uneven, or the probe tip subscribes an arc when the chip comes into contact with the probe or probes effecting scratching of the chip surface resulting in damage to the delicate electrical circuits therein. In other instances, uncontrolled or poorly controlled probe tip forces on the chip surface can result in damage to either or both of the chip surface and probe tip.
Other methods of control of the probe arm is to provide an actuator for each of the probes and move the chip under test only in the X-Y plane. Some of the actuators employed include that shown in U.S. Pat. No. 5,153,472. While this probe actuator overcomes the problems of non linear probe movement in the Z-axis, and meets the requirements of controllable probe force, it suffers from two major defects: (1) the ball bearing structure is subject to wear and stress concentration, resulting in non-repetitive or repeatable accurate alignment of the probe tip over a period of time, and (2) the high mass of the tip armature structure makes probe tip control difficult unless probe movement is deliberately made slow so as to prevent inadvertent high impact loads on the chip. Other actuator designs have included air bearings for their movement. However, air bearings are not stable at high speeds because of turbulent flow.
Other problems relating to electrical signal interference when measuring with the probe tip, using electrically powered actuators, are overcome as shown in U.S. Pat. No. 4,123,706 by the use of fluid actuation. However, it is believed that the twin beam actuator shown in the '706 patent has damping problems with fast probing contacts moving at high speeds. Moreover, despite the characterization of "no arc" movement of the probe, it is believed that the probe tip of the '706 patent has to move in some arc which creates some difficulty in initial positioning of the actuator/probe tip, especially when the chip surface is uneven, or the alignment of chip surface and probe tip is not perfect. Also, because the principal actuation is in one direction or uni-sided, probe bounce may occur at high testing speeds and force.